中文摘要：

采用双向循环恒刚度剪切试验对预制桩的侧阻退化效应进行试验研究。研究显示，剪切应力（摩阻力）随剪切循环数的增加呈指数型衰减，衰减主要发生在开始的部分循环内，约25个循环后基本达到稳定。剪切过程中剪切带发生明显的剪缩，导致法向应力释放，此为摩阻力退化的原因之一。随剪切循环数的增加界面摩擦角发生指数型退化直至达到残余值，此为摩阻力退化的另一原因。法向刚度的大小决定剪切应力、法向应力和界面摩擦角衰减的速度和幅度，法向刚度越大，衰减越快且残余值越小。恒刚度剪切试验说明桩土界面摩擦力的退化与桩周土的坚硬程度密切相关，土体越硬则侧阻退化现象越明显。

英文摘要：

A series of constant normal stiffness direct shear tests are performed to investigate the friction fatigue for preformed piles. The results show that the shear stress （friction resistance） displays a exponential decay trend with the increasing shear cycle number, and most of the reduction occurs during the first few cycles and reaches a steady value after about 25 cycles. Cyclic shears induce a contraction of the narrow shear zone, leading to a release of normal stress, which is one of the causes for the friction fatigue. The interface friction angle decreases exponentially with the shear cycles within the first few cycles and subsequently approaches a residual value, which is another cause for the friction fatigue. The normal stiffness controls the degradation rates and degradation extents of the shear stress, normal stress and interface friction angle. A larger normal stiffness causes a faster degradation and a smaller residual value. The above results indicate that the pile-soil friction fatigue is closely related to the stiffness of the surrounding soil, and the stiffer soil will lead to a more evident friction fatigue effect.